1. Field of the Invention
This invention pertains to regulating apparatus for lighting control systems and more specifically to automatic regulating apparatus for adjusting a lighting system to compensate for the amount of natural daylight or other external light illuminating the work area.
2. Description of the Prior Art
It is frequently desirable for a given work area to receive a desired amount of uniform light intensity even though the individual light from various sources providing illumination of the area may change. For example, if power to the primary lighting system fails, it is desirable to have a backup or standby power supply so that light to the illuminated area can be continuous. If a light burns out or the circuit to a light system fails, it is desirable to have an auxiliary light or lighting system to provide continuous lighting.
Most work areas are provided natural light or daylight through windows to the outside as well as from an internal artificial lighting system. Although rheostats and more sophisticated dimming devices have been around for quite some time, they generally have provided only manual control of the internal lighting system so that, for instance, the lights can be turned up at night when the sun goes down.
More recently, lighting systems have been combined with microprocessors so that lights can be connected, for example, to a time clock for varying the amount of light during particular periods of time. Such a system would work quite satisfactorily to provide a change of light upon the arrival of night time hours, but would not change the amount of light because there was a storm outside, which might be just as darkening of the work area as the sun going down.
It has been known that it might be possible to automatically control the amount of voltage to a lamp driver using a photosensitive device. That is, a typical photosensor produces a voltage proportional to or inversely proportional to the sensed light intensity. By using this voltage to drive a lamp system, it would seem that it would be easy to compensate for variation in the externally sensed light. That is, as the external light dims, the inside light would be made brighter. As the external light becomes brighter, then the inside light would be made dimmer.
However, the light intensity from the sun varies over an extremely wide dynamic range, on the order of illuminating an area with an intensity of from about 10.sup.4 foot candles to 10.sup.-5 foot candles. The eye perceives a logarithmic change of light intensity as a linear change of brightness. Therefore, a proportionally linear change of voltage with light intensity would not produce the desired result of the control system. An attempt has been made to produce a non-linear drive voltage to the lamp circuit using non-linear potentiometric resistors and an operational amplifier connected as a comparator. The theory is that the ratio of the bias resistor to feedback resistor would result in a non-linear drive voltage from the comparator for connection to the lamp or lamp system. However, it is virtually impossible to reliably synchronize non-linear potentiometers and therefore such a theoretical control regulator has not proven to be commercially feasible.
Therefore, it is a feature of this invention to provide an improved photocontrolled apparatus suitable for compensating a lighting system to compensate for variation in the light intensity of an external light illuminating the same area as such lighting system.
It is another feature of this invention to provide an improved lighting control system incorporating a logarithmic-to-linear device in such a manner to compensate for the natural logarithmic change of lighting that occurs from the sun, which is over a wide dynamic range.